1. Field of the Invention
The present invention relates to an information recording and/or reproducing apparatus and an optical pickup device. More particularly, the present invention relates to an information recording and/or reproducing apparatus and an optical pickup device, each of which uses a solid immersion lens.
2. Description of the Related Art
In order to read digital signal information from and write digital signal information onto a related optical recording medium, such as a digital versatile disc (DVD), an optical pickup device comprising a semiconductor laser and an objective lens as main components is used. The digital signal information is stored in the form of fine pits on the disc. The semiconductor laser emits laser beams having wavelengths in the visible range. The objective lens has a predetermined numerical aperture. For example, when a DVD-ROM (read-only-memory) having a recording capacity of 4.7 GByte is used, laser beams are converged on an information recording surface of the disc, using a red semiconductor laser, which emits light having a wavelength of 650 nm, and an objective lens having a numerical aperture (NA) of 0.6; and changes with time in the strength of detection of returning light reflected from the information recording surface is converted into an RF signal waveform in order to reproduce information recorded on the disc. A DVD-ROM disc has a plurality of pits formed with a depth of approximately 110 nm and a track-pitch interval of 0.74 μm. A pit length and an interval from one pit to a succeeding pit are from a minimum of 0.4 μm to 1.47 μm. The DVD-ROM disc has spiral tracks.
In recent years, for a laser light source of an optical pickup device, a GaN semiconductor lasers (LD) which oscillates at wavelengths from the blue light range to the ultraviolet range has been rapidly developed. An attempt is being made to use such a GaN semiconductor laser in a next-generation optical disc system for a DVD.
A practical shortest oscillation wavelength of a GaN semiconductor laser is approximately 350 nm. Due to a band gap that is determined by a material used, it is difficult to make the wavelength shorter than this. It is expected that an objective lens having an NA of approximately 0.85 that is much higher than the NA of an objective lens based on a DVD standard or used in a recording and/or reproducing apparatus based on the DVD standard will be used in the next-generation optical disc system. In this way, in recent years, the use of an objective lens having an NA as high as that of a microscope in the next-generation optical disc system is being considered. However, as long as an objective lens is used in a recording and/or reproducing system in what is called a far field optical system that is similar to a related optical system, there is a limit as to how high the NA can be made, which is approximately 0.95.
With the rapid development of information communication and image processing technologies in recent years, in the future, an optical disc will be required to have a sub-terabyte storage capacity like that of a hard disc (HDD) whose storage capacity has been considerably increased in recent years. Here, a method for realizing a high recording density and high storage capacity in an optical recording medium having, for example, the same size and recording mode as a DVD is considered.
For an optical pickup device comprising a GaN semiconductor having a shortest oscillation wavelength of approximately 350 nm (mentioned above) and an objective lens having a maximum NA of approximately 0.95 (mentioned above), when a simple proportional calculation based on parameters of a DVD optical pickup device is carried out, a recordable/reproducible density (capacity) of an optical disc, serving as an optical recording medium, is: ((650 nm/350 nm)×(0.95/0.6))2×4.7 GByte=41 GByte
Therefore, as long as such an optical pickup device is used, when, for example, a DVD of the same size and recording mode is used, a recording capacity of approximately only 41 GByte for a single layer can only be obtained. Consequently, it is difficult to achieve the object of increasing the recording density and storage capacity of an optical recording medium with the rapid development of information communication and image processing technologies in recent years.